Quaternary stereophonic sound reproduction apparatus

ABSTRACT

A quadraphonic sound reproduction has a apparatus decoder for restoring encoded signals to four signals in four audio channels, and a circuit for comparing the component of the signal in each channel with a leakage component in that channel into which the signal leaks the least to produce a balancing signal according to the ratio of the amplitudes of the two components. A circuit is provided for applying the balancing signal to the remaining two channels to balance out crosstalk components of the signal developed in these two channels.

United States I Patent [191 Hiramatsu Aug. 13, 1974 QUATERNARY STEREOPHONIC SOUND REPRODUCTION APPARATUS [75] Inventor: Katsuzo Hiramatsu, Koriyama,

Japan [73] Assignee: Mitsubishi Denki Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Oct. 4, 1972 [2l] Appl. N0.: 297,031

[52] U.S. Cl. 179/1 GQ, 179/100.1 TD, l79/100.4 ST [51] Int. Cl. H04n 5/00 [58] Field of Search 1 79/) G l (3Q. BI,

179/100.4 ST, 100.1 TD

[56] References Cited UNITED STATES PATENTS 3,632,886 l/l972 Scheiber 179/15 BT 3,708,631 l/l973 Bauer 179/1 GQ 3,786.193 l/l974 Tsurushima 179/] GO OTHER PUBLICATIONS Multichannel Stero Matrix Systems, An Overview by Eargle, Journal ABS July/August l9 7l, pp. 556, 557.

Discrete vs. SQ Matrix Quadraphonic Disc by Bauer, Audio Magazine July 1972.

The New Sansui, db Matrix, Audio July 1972, p. 38.

Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas DAmico Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [57] ABSTRACT A quadraphonic sound reproduction has a apparatus decoder for restoring encoded signals to four signals i n 6 Claims, 8 Drawing Figures IN TERM CH llN IN TERM cH 2m OUT TERM N TERM CH3|N PHASE CHioUT N TERM CH4|N cpl 9H2 c143 CIH4 1 210M INVERTER NO.| 3o MIX R N0.1

' 1 l l l 1 COM 8 DET CKT No.1 TR4\ 0p EAL 816 GENERATOR 22 TRI l 2 lXER No.2

T l 3o BAL SIG GENERATOR PHASE OUT TERM COM a BET CKT No.2 0 T NO. 2 INVERTER, L CHZOUT No. 2 22 I L? 4 p 2-3 5% l 22 L gMlXER N03 BAL SIG GENERATOR PHASE '3-2 OUT TERM 26 coM a DET CKT N05 0% N0 3 INVERTER CH 30 t 32 No.3 g 2 LP L MIXER N04 BAL SIG GENERATOR PHASE 'fi TERM 26 COM 6. DET CKT N0. 4 044 NO 4 INVERTER 3' CH UT 1 N04 LR T .E

PATENTEDIIUBI 3M4 3.829.615

SIEH 101 3 IO I (D/IN TERM CH I|N OUT TERM AMPL 0 CHIOUT FIG. 1 I PRIOR ART D W J T E i CH IN' v OUT TERM O AMPL 2 CH ZOUT D E 16 R IN O T TE M TERM CH 3 U R )5 IN AMPL Q CH 3 l 18 IN I iERM CH 4IN OUT TERM L OUT 2iO 4 I COMP 8 OET CKT PHASE (INVERTER [24I-4 |4 CONTROL CONTROL GENERATION GENERATION cIRcuIT CIRCUIT FIG, 2 FIG "3 SOUND REPRODUCER ARL SOUND A RL ARR REPRODUCER AR AR AFR O11 O13 AFL L AR C 1 8 CH3 R O O 1 R O O Y F FR FL R LISTENER LIsTENE R G O AFR O C) 2 2 I I 2 CH2 CH4 -AFR I CH2 CH4 L AFR R R R L ARL. R AF AFL RL RR PAIENIE AU!) 1 31am I sum 2 as a SOUND SOUND REPRODUCER REPRODUCER CH) (i -1 3 H 1 H3 LISTENER USTENER 5 3 I CH2 CH4 CH2 CH4 I ARR R AFL FIG. 40 FIG. 4b

EPSSSBQER R FIG. 50 CH1 CH3 20 4 O i a) coMPa FL AFL RT 28 -4;. V Ni O BAL I? SIG CH2 CH GENERATOR A'FL HH- PHASE INVERTER FIG. 5b

SOUND RERRODUQE s H A'RR 35R RR CKT J 4-| v BAL. SIG. CH2 CH4 GENERATOR TAT-RR- 1 2.2

PHASE INVERTER 1 QUATERNARY STEREOPHONIC SOUND REPRODUCTION APPARATUS BACKGROUND OF THE INVENTION This invention relates to improvements in a quadrophonic sound reproduction system utilizing a matrix and more particularly to means for increasing the degree to which a signal in each of four audio channels of such a sound reproduction system is acoustically separated from the signals in the other channels without making the auditory sensation unnatural.

In order to establish sound fields to give the feeling that the audience is present in the music hall, multidirectional acoustic systems are presently being changed from the two channel type to the four channel type. The four channel acoustic systems presently employed are matrix, discrete, and pseudo-four channel systems. Among them the matrix system, to which the invention relates, is subdivided into the 2-2-4 channel system and the 4-2-4 channel system depending on the transmission process involved. The 2-2-4 channel system attempts to establish attractive sound fields giving the feeling that the audience is present in the music hall, which fields have previously not been established, by picking up only the ambience information hidden in the source of sound for the conventional two channel stereophonic system and reproducing the ambience information from points to the rear or side of the listener relative to the associated sound reproduction apparatus. On the other hand, the 4-2-4 channel system is generally operative to pick up four signals from a source of sound through the use of four separate microphones, to transfer the picked-up signals to the two channel transmission system to compress them to a two channel format by an encoder and restoring the components of the signal thus compressed to the four original signals by a decoder in order to reproduce them separately. The 4-2-4 channel system is described and claimed in US. Pat. No. 3,632,886.

ln matrix type sound reproduction apparatus for use in the 4-2-4 channel system, part of the signal in the reference audio channel in which the desired signal is developed tends to leak out into the adjacent audio channels with the result that the degree of acoustic separation between the audio channels decreases. In other words, cross talk inevitably occurs between the audio channels. In order to prevent the occurrence of such cross talk between the channels, there have been previously proposed logic circuits for comparing the desired signal in each channel with a cross talk component thereof developed in the diagonally opposite channel corresponding to diagonally opposite speakers into which a minimum amount of the desired component of the signal, in the terms of the amplitude, leaks to form a difference signal therebetween for controlling the gains of amplifiers connected in the remaining or flanking channels respectively. Where that measure has been used to reproduce a particular piece of live music, the presence of any relatively low frequency portion of the musical signal in one of the flanking channels has caused a decrease in the gain of the amplifier disposed in that one channel so that a portion of the musical signal fails to be reproduced resulting in a loss of transmitted information. Also the reproduction of live music is usually accompanied by the occurrence of continuous noise other than the reproduced musical signal, for example, reverberation tones. Thus the use of the logic circuits as above described can cause the disappearance or reduction of that noise simultaneously with the reproduction of the musical signal resulting in a disadvantage that the auditory sensation becomes unnatural.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention to provide a new and improved four channel type stereophonic sound reproduction apparatus for increasing the degree of acoustic separation between each pair of four audio channels by effectively eliminating cross talk occurring between each pair of channels. It is another object of the invention to provide a new and improved four channel type stereophonic sound reproduction apparatus for eliminating loss of transmitted information or an unnatural auditory sensation while increasing the degree of acoustic separation between each pair of flanking audio channels involved.

The invention accomplishes these objects by providing a quadraphonic sound apparatus for use in a quaternary stereophonic sound recording and reproducing system including encoder and decoder means, comprising reproducing decoder means for restoring components of a signal compressed to a two channel format to components of the signal developed in four audio channels, and means for comparing the restored signal in each of the four audio channels with a cross talk component thereof in a selected one of the other audio channels in terms of the amplitude thereof to produce a control signal, characterized in that the control signal is a balancing signal dependent upon the ratio of the amplitude of the restored signal to the cross talk component thereof as detected by the comparison means, and means for applying the balancing signal to the remaining two channels to attenuate cross talk components of the signal developed therein whereby the degree of acoustic separation between the audio channels is increased.

The means for attenuating the leakage components of the signal preferably includes a phase inverter circuit for inverting the phase angle of the balancing signal and a mixer circuit connected to both the phase inverter circuit and a selected one of the four audio channels to add the balancing signal to the cross talk component of the signal in the selected audio channel in phase inverted relationship.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of a device for increasing the acoustic separation between the four audio channels in a matrix type four channel stereophonic sound reproduction systems constructed in accordance with the principles of the prior art;

FIG. 2 is a diagrammatic view illustrating one form of correlation among input signals to the four audio channels in matrix type four channel stereophonic sound systems used in both the prior art and the invention;

FIG. 3 is a diagrammatic view similar to FIG. 2 but illustrating another form of the correlation among the input signals applicable to the invention;

FIGS. 4a and 4b are diagrammatic views useful in explaining signals developed in the four audio channels with the original signal including only a single component fulfilling the correlation among the input signals shown in FIG. 2;

FIGS. 5a and 5b are block diagrams useful in explaining the principles of the invention; and

FIG. 6 is a combined circuit and block diagram of an embodiment of the acoustic separation increasing device of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. 1 in particular, it is seenthat the arrangement disclosed herein comprises a reproducing decoder 10 for restoring a pair of signals into which an input signal has been encoded to a two channel signal back to the four original signals which are, in turn, developed at four terminals which are connected to four input channels CH1 CH2 CI-I3 and CH4, of a reproducing device. Four variable gain amplifiers 12, 14, 16 and 18 are connected in the four audio channels CH1, CH2, CH3 and CH4 between said input terminals CI-Il CHZ CH3 and CH4 and output terminals CHI CH2 CH3 and CH4 respectively. The input terminals CH1,- and CH4 are connected to two inputs to a combined comparison and detection circuit 20 for comparing the restored signal developed in the channel CH1 with a cross talk component thereof developed in the channel CH4 in terms of the amplitudes of the signal and the component to detect a difference signal therebetween. If channel CH1 in which the restored signal is developed is called a reference channel the channel CH4 is called the diagonally opposite channel. The term diagonally opposite channel means that channel into which the cross talk component of the signal in the reference channcel leaks to a minimum degree. The channels CH1 and CH4 serve as detection channels.

The difference signal from the circuit 20 is applied to a phase inverter circuit 22 where the phase thereof is inverted. The phase inverter circuit 22 is adapted to provide a pair of signals having the phases thereof inverted from each other which signals are supplied to individual control generation circuits 24 and 26 to generate a pair of control signals respectively. The control signal from the circuit 24 is applied to the variable gain amplifiers 14 and 16 connected in channels CH2 and CH3 to control the gains thereof respectively. Similarly the control signal from the circuit 26 is applied to the variable gain amplifiers 12 and 18 connected in channels CH1 and CH4 respectively for the purpose of controlling the gains. Thus the amplifiers 12 through 18 amplify the signals in the channels CH1, CH2, CH3 and CH4 with the respective gains as determined by the control signals applied thereto respectively.

The operation of the arrangement as shown in FIG. 1 will now be described in conjunction with FIG. 2. Referring to FIG. 2, a central square block represents a quadraphonic sound reproduction apparatus and a listener is designated by the central circle with the arrow within the block who can simultaneously hear acoustic information from four points represented by circles denoted adjacent the corners of the square block and labelled CH1, CH2, CH3 and CH4. The acoustic information from each of those points is what has been transmitted through the respective channels labelled with the same reference characters (see FIG. 1). For example, the acoustic information from the point CH1 is what has been transmitted through the channel CHI.

Various components of the signal capable of reaching the points CH1, CH2, CH3 and CH4 respectively are designated by vectors adjacent the associated points or circles outside the block. The reference characters F, R, L or R beside the vectors represents that side of a source of signal on which a signal is picked up by an individual microphone. That is, the reference characters F and R represent the front and rear of the source respectively while the characters L and R designate the left and right of the source respectively. For example, a vector F represents a component of a signal picked up on the left side of the source in the front thereof. Then that component of the signal is transmitted through the channel CH1 until is appears at the point as shown by CH1 in FIG. 2. Also the signal can include another component as represented by a vector R picked up on the right side and in the rear of the source and appearing at the point CH4 (see FIG. 2) through the channel CH4 as shown in FIG. 1.

Further any vector prefixed with the Greek letter A designates a cross talk component due to the restored component of the signal represented by the vector shown by the reference characters following the A. For example, AR means a cross talk component resulting from the restored signal R and shown as being developed in the channel CH1.

The vectors each have a length representing the amplitude of the corresponding component of the signal and are at an angle with the adjacent vector providing a phase angle therebetween. For example, in the channel CH1, the vector F is at a phase angle of with respect to the vector AR and is in phase with the vector AF Assuming that among the components of the original signal, there is included only a single component F the decoder 10 on the reproducing side provides a corresponding component F L of the reproduced signal for the reference channel CH1 shown by the double circle in FIG. 4a. Under these circumstances, cross talk components AF L of the signal will be developed in the channels CH2 and CH3 as shown in FIG. 4a and a difference in amplitude between the components of the signal developed in the detection channels or the reference channel CH1 and the diagonally opposite channel CH4 (which is shown by the dotted circle in FIG. 4a) will be come large. As above described, the combined comparison and detection circuit 20 compares the components of the signal developed in both channels CH1 and CH4 to provide a difference signal therebetween. The difference signal is processed by the phase inverter and control generator circuits 22 and 221-4 respectively in the manner as above described to form a control signal. The control signal is supplied to the amplifiers l4 and 16 connected respectively in the channels CH2 and CH3 to decrease the gains thereof. Therefore those components AF of the signal leaking into the channels CH2 and CH3 called hereinafter flanking channels and developed at the outputs of the amplifiers 14 and 16 further decrease in amplitude with the result that the component F L of signal in the channel CH1 appears in a correspondingly accentuated state. The term flanking channel means any one of the channels other than the reference and diagonally opposite channels.

Assuming now that the original signal includes only a single component F it will readily be understood that a corresponding component F R of the reproduced signal is developed in the channel CH3 while cross talk components thereof AF R are developed in the channels CH1 and CH4 flanking to the channel CH3. In that event the component of the signal in the channel CH1 becomes as large in amplitude as that in the channel CH4. Then circuits for CH2 and CH3 corresponding to circuits 20 22 24 and 26 (not shown) are operated in a similar manner as above described for the presence of the signal component F alone excepting that the control signal provided by the circuit corresponding to circuit 26 is inverted in phase from that above described. The circuit 20 senses the amplitudes of the signals applied to the two inputs thereto to produce an output. More specifically if the signals at the two inputs differ in amplitude then an output having a certain magnitude. is produced at the output of the circuit 20. On the other hand, if those signals have substantially equal amplitudes then a null output is produced from the circuit 20.

The circuit 22 includes one input and two outputs. When the circuit 20 applies the signal having the certain magnitude to the input to the circuit 22, a signal is output from one of the two output of the circuit 22. However, when the circuit 22 receives a null output from the circuit 20, a signal is developed at the other output of the circuit 22.

Then the one output from circuit 22 is applied to the circuit 24 while the other output from the circuit 22 is applied to the circuit 26. Then the control signal is supplied to the amplifiers l2 and 18 connected respectively in the channels CH1 and CH4 to decrease their gains. This similarly causes the component F n of the signal in the channel CH3 to be relatively accentuated.

The process as above described is repeated with each of the other components of the original input signal present on the transmitting side. For example, a component R alone included in the original signal is reflected in both a corresponding component R of the signal R and leakage components thereof AR developed in the channel CH4 and the channels CH2 and CH3 respectively as shown in FIG. 4b using the same reference characters and symbols as in FIG. 4a. In that event, the amplifiers l4 and 16 connected respectively in the channels CH2 and CH3 are controlled to decrease the gain.

The operation of the arrangement as shown in FIG. 1 is summarized as follows: The signal restored in each of the audio channels reproduced by the decoder is compared with a cross talk component thereof developed in the diagonally opposite audio channel in terms of the amplitudes thereof to provide a control signal. Then the control signal is utilized to control the gains of those amplifiers connected respectively in the flanking channels, whereby the acoustic separation is increase through the relative accentuation of the restored component of the signal.

If the arrangement of FIG. 1 is used to reproduce particular live music then the presence of any relatively low frequency music signal in any one of the audio channels called the flanking channels causes the gain of the associated amplifier to decrease thereby reducing the capability of that music signal to be reproduced resulting in a loss of transmitted information. Also as Well known, the reproduction of live music is usually accompanied by the occurrence of continuous noise such as reverberation tones. Such noise will at least partly disappear or be reduced along with the coexistent portions of the music signal resulting in a disadvantage that the auditory sensation becomes unnatural.

The invention foregoing elimination of the disadvantage as above described.

The fundamental concept of the invention is to erase the cross talk components of the reproduced signal per se without decreasing the gains of amplifiers in those audio channels in which cross talk components of the signal are developed as in the arrangement as shown in FIG. 1.

Referring now to FIG. 5a, there is illustrated a device for increasing the acoustic separation between the four audio channels in quadraphonic sound reproduction systems constructed in accordance with the principles of the invention. It is assumed that the arrangement illustrated has applied thereto inputs as shown in FIG. 2. It is also assumed that the original signal includes only a single signal F Under the assumed condition, a corresponding signal F reproduced in the reproducing side is developed in the audio channel CH1 and cross talk components thereof AF L are developed in the channels CH3 and CH2.

A combined comparison and detection circuit 20 is shown in FIG. 5a as having inputs applied thereto from the reference and diagonally opposite channels CH1 and CH4 as shown at double and dotted circles in FIG. 50 respectively. As in the arrangement of FIG. 1, the circuit 20 is operative to provide a control signal as a result of the comparison of the components of the signal in both channels but dependent upon the signal ratio between the channels CH1 and CH4. The control signal is applied to a balancing signal generation circuit 28 This circuit 28 responds to the applied control signal to generate a balancing signal AF After having had the phase thereof inverted by the phase inverter circuit 2 the balancing signal now designated by -A'F is applied to the flanking channels CH2 and CH3 to be addedin reversed phase relationship to the cross talk components AF of the signal leaking out thereinto as shown in FIG. 5a. This results in the balance-out or attenuation of the cross talk components of the signal developed in the channels CH2 and CH3.

Where the components of the signal are developed in the respective channels as shown in FIG. 2 if the original signal includes only a single signal labelled R then the cross talk components thereof are balanced out or attenuated in a similar manner to that described in conjunction with FIG. 5a. As shown in FIG. 5b wherein like reference characters with different subscripts designate components corresponding to those illustrated in FIG. 5a, the channels CH4 and CH1 are the reference and diagonally opposite channels respectively and the flanking channels CH2 and CH3 have developed therein cross talk components of the signal reversed in phase from each other and designated by AR and AR',;. Therefore in order to attenuate those cross talk components of the signal, a pair of balancing signals reversed in phase from each other are required. To this end, the phase inverter circuit 22 should provide a pair of balancing signals A'R and 7 AR These balancing signals -A'R and AR are applied to the channels CH2 and CH3 in such directions that the cross talk components AR and AR are balanced out or attenuated.

Where the original signal includes only a single signal P or R the process as above described is repeated by other comparison and detection circuits, balancing signal generation circuits and phase inverter circuits (not shown) to balance out or attenuate the associated cross talk components of the reproduced signal with the reference and diagonally opposite channels suitably changed.

It is to be noted that the amplitudes of balancing signals AF A'R etc. should be so as not to exceed those cross talk components AF AR etc. to be balanced out.

In FIG. 6 wherein like reference numerals designate the components corresponding to those shown in FIG.

5 or 1, there is illustrated one form of the invention operative for an original signal including at least two signals. The arrangement illustrated comprises four series combinations of comparison and detection circuits 20, balancing signal generation circuits 28, phase inverter circuits 22 and mixer circuits 30 connected in parallel circuit relationship between four input terminals CH1, CH2 CH3 and CH4, and the associated output terminals CHI CHZ CH3 and CH4 to form four audio channels CH1, CH2, CH3 and CH4.

More specifically, each of the combined comparison and detection circuits 20 is connected across a different pair of input terminals of the audio channels and identical in operation to the circuit 20 as shown in FIG. 5. For example, the circuit 20,., is connected across the input terminals CH1 and CH4,, to compare the signal developed in the channel CH1, in this case, the reference channel and the cross talk component thereof developed in the channel CH4 or in the diagonally opposite channel and detect a signal ratio therebetween.

All the circuits 20 designated Nos. 1, 2, 3 and 4, are of the same construction and the circuit 20 will now be described. As shown in F IG. 6, the circuit 20 comprises two pairs of directly coupled NPN transistors TRl, TR2 and TR3, TR4 and associated capacitors, diodes and resistors disposed substantially symmetrically with respect to the output 0 in the manner illustrated. The transistors TR2 and TR4 include bases operatively coupled to the input terminals CH1 and CH4, through the respective inputs.

Each of the circuits 20 is connected through the output thereof to one input to the corresponding balancing signal generation circuit 28 which has the other input connected to the reference channel for the corresponding circuit 20 and thence to the corresponding phase inverter circuit 22.

All the circuits 28 designated Nos. 1, 2, 3 and 4 are the same in operation as the circuit 28 shown in FIG. 5 and are identical in construction to one another and the circuit 28 will now be described. This circuit comprises an NPN transistor TRS having a base operatively coupled to said other input, i.e. the input terminal CH1, or the reference channel for the circuit 20 and a unijunction transistor UT having a base connected to the output 0, of the associated circuit 20 through the one input. Both transistors TRS and UT form a gain control circuit with the associated resistors capacitors and a diode connected to one another and .to the transistors in the manner illustrated. As above described, the circuit 28, generates the balancing sig nal such as A'F which is, in turn, applied to the phase inverter circuit 22 All the phase inverter circuits 22 designated Nos. 1,

2, 3 and 4 are identical in construction to one another and each circuit comprises an NPN transistor TR6 and the associated resistors and capacitor connected to one another and to the transistor in the manner illustrated. The transistor TR6 has a base operatively coupled to the collector of the transistor TRS, a collector connected to one output and an emitter connected to the other output of the phase inverter circuit. The inverter circuit is adapted to supply either one or both of the balancing signals in phase and out of phase by through the collector and emitter of the transistor TR6 to the succeeding mixer circuit 30. In FIG. 6, the inverter circuits 22 designated Nos. 1 and 2 are shown as supplying the balancing signal through the emitter of the transistor to the succeeding mixer circuit 30 and those designated Nos. 3 and 4 are each adapted to provide a pair of balancing signals.

Four mixer circuits 30 of the same construction denoted Nos. 1, 2, 3 and 4 are connected in the audio channels CH1, CH2, CH3 and CH4 respectively. Each of the mixer circuits 30 comprises an NPN transistor TR7 having connected thereto resistors and capacitors in the manner illustrated. The transistor TR7 includes a base operatively coupled to three parallel resistors forming three inputs and a collector supplying an output to the associated output terminal CH 1 CHZ C113 or CH4 A first one of those inputs is connected one of the audio channels CH1, CH2, CH3 or CH4, and the remaining inputs are connected to a different pair of the phase inverter circuits 22 so that the cross talk component of the signal in the associated channel is attenuated.

From FIG. 6 it will be seen that one of the combined comparison and detection circuits 20 is provided for each pair of detection channels or each set of the reference and diagonally opposite channels. The pair of the detection channels are determined by the type of the restored component of the signal while at the same time the flanking channels to be supplied with the balancing signal are changed accordingly. Thus the cross talk components of the signal are effectively balanced out for each type of the restored component with the result that the acoustic separation between the audio channels is increased.

More specifically, with the components of the signal selectively developed in the four channels as shown in FIG. 2, the diagonally opposite channel is located on the diagonal of the square block as shown in FIG. 2 having the reference channel disposed thereon and the flanking channels are those next to the reference channel. Thus if the reference channel is the channel CH1 then the diagonally opposite channel is the channel CH4 and the channels CH2 and CH4 are called the flanking channels. This is apparent from the fact that the diagonally opposite channel is that channel into which the restored component of the signal from the reference channel leaks to the least extent and that the flanking channels designate the channels other than the reference and diagonally opposite channels. In the example just described the combined comparison and detection circuit has inputs applied from the channels CH1 and CH3 with the balancing signal applied to the channels CH2 and CH4.

From the foregiong it will be appreciated that the invention makes it possible to increase the acoustic separation between the audio channels without both a loss of transmitted information and an unnatural auditory sensation otherwide occurring in the prior art type devices.

While the invention has been illustrated and described in conjunction with a single preferred embodiment thereof it is to be understood that various changes and modifications may be resorted to without departing from the spirit and scope of the invention. For example, one of the combined comparison and detection circuits and 20 may, if desired, be omitted because both devices compare the components of the signal developed in the same channels CH1 and CH4 with each other. This is true in the case of the two circuits 20 and 20 connected to the channels CH2 and CH3. With one of the circuits 20 thus omitted, the balancing signal generation circuit 28 connected to the omitted circuit 20 is also omitted.

What is claimed is:

1. A quadraphonic sound reproduction apparatus for use in quadraphonic sound recording and reproducing systems comprising in combination, reproducing decoder means for restoring encoded signals to four signals, four audio channels coupled to said reproducing decoder means, a plurality of means, each coupled to a pair of said audio channels corresponding to oppositely located speakers for comparingthe amplitude of said restored signal of the signal in one of said pair of audio channels with the amplitude of a cross talk component thereof in the respective other audio channel, each of said means producing a balancing signal from said restored signal the magnitude of which is dependent upon the ratio of the amplitudes of the restored signal and the cross talk component of the respective signal, and attenuating means coupled between each of said comparing means and the other two audio channels for applying said balancing signal from said comparing means to the remaining two channels to attenuate cross talk components of the signal developed therein by applying a signal opposite in phase to said cross talk, whereby the acoustic separation between the audio channels is increased.

2. A quadraphonic sound reproduction apparatus as claimed in claim 1 wherein said means for attenuating the cross talk component of the signal includes a phase inverter circuit for inverting the phase of the balancing signal, and a mixer circuit connected to said phase inverter circuit to add said balancing signal to the cross talk component of the signal in phase inverted relationship, thereby eliminating the cross talk component.

3. A quadraphonic sound reproduction apparatus as claimed in claim 1 wherein said comparing means includes means for generating the balancing signal which comprises a detection circuit for producing a control signal in accordance with the ratio of the amplitude of said restored signal and said cross talk component of the signal in the selected audio channel, and a gain control circuit coupled to the detection circuit and responsive to the applied control signal to produce the balancing signal for eliminating the cross talk component of the signal.

4. A quadraphonic sound reproduction apparatus for use in quadraphonic sound recording and reproducing systems comprising in combination, reproducing decoder means for restoring encoded signals to four signals, four audio channels coupled to said reproducing decoder means each channel receiving one signal therefrom, a plurality of combined comparison and detection circuit means each coupled to respective pairs of said audio channels, one of each of said pairs being a reference channel and the other being an associated audio channel said two channels being connected to diagonally opposite speakers, for comparing said restored signal in each of the respective reference audio channels with a cross talk component of the signal developed in the associated channel in terms of the amplitude of the signal and the cross talk component, and for detecting the ratio of the amplitude of said restored signal to said cross talk component, a plurality of balancing signal generation circuit means each coupled to one of said combined comparison and detection circuit means and to said respective reference audio channel for generating a balancing signal from said restored signal in said respective reference audio channel in accordance with said ratio, and a plurality of attenuating circuit means each coupled to one of said balancing signal generation circuit means and to audio channels corresponding to speakers flanking said reference channel speaker for applying the balancing signal to each of the flanking audio channels to attenuate the cross talk component of the signal resulting from the restored signal in each of the audio channels by applying a signal opposite in phase to said cross talk, whereby the cross talk component is eliminated for increasing the acoustic separation between the audio channels.

5. A quadrophonic sound reproduction apparatus as claimed in claim 4 wherein said attenuating circuit means includes a phase inverter circuit for phase inverting the balancing signal and a mixer circuit connected to said phase inverter circuit to add the phase inverted balancing signal, to the cross talk component of the signal developed in the respective audio channels corresponding to flanking speakers, thereby eliminating said cross talk.

6. A quadraphonic sound reproduction apparatus as claimed in claim 4, wherein said associated audio channel in a pair of audio channels is that audio channel into which said restored component of the signal from the reference channel does not ideally leak. 

1. A quadraphonic sound reproduction apparatus for use in quadraphonic sound recording and reproducing systems comprising in combination, reproducing decoder means for restoring encoded signals to four signals, four audio channels coupled to said reproducing decoder means, a plurality of means, each coupled to a pair of said audio channels corresponding to oppositely located speakers for comparing the amplitude of said restored signal of the signal in one of said pair of audio channels with the amplitude of a cross talk component thereof in the respective other audio channel, each of said means producing a balancing signal from said restored signal the magnitude of which is dependent upon the ratio of the amplitudes of the restored signal and the cross talk component of the respective signal, and attenuating means coupled between each of said comparing means and the other two audio channels for applying said balancing signal from said comparing means to the remaining two channels to attenuate cross talk components of the signal developed therein by applying a signal opposite in phase to sAid cross talk, whereby the acoustic separation between the audio channels is increased.
 2. A quadraphonic sound reproduction apparatus as claimed in claim 1 wherein said means for attenuating the cross talk component of the signal includes a phase inverter circuit for inverting the phase of the balancing signal, and a mixer circuit connected to said phase inverter circuit to add said balancing signal to the cross talk component of the signal in phase inverted relationship, thereby eliminating the cross talk component.
 3. A quadraphonic sound reproduction apparatus as claimed in claim 1 wherein said comparing means includes means for generating the balancing signal which comprises a detection circuit for producing a control signal in accordance with the ratio of the amplitude of said restored signal and said cross talk component of the signal in the selected audio channel, and a gain control circuit coupled to the detection circuit and responsive to the applied control signal to produce the balancing signal for eliminating the cross talk component of the signal.
 4. A quadraphonic sound reproduction apparatus for use in quadraphonic sound recording and reproducing systems comprising in combination, reproducing decoder means for restoring encoded signals to four signals, four audio channels coupled to said reproducing decoder means each channel receiving one signal therefrom, a plurality of combined comparison and detection circuit means each coupled to respective pairs of said audio channels, one of each of said pairs being a reference channel and the other being an associated audio channel said two channels being connected to diagonally opposite speakers, for comparing said restored signal in each of the respective reference audio channels with a cross talk component of the signal developed in the associated channel in terms of the amplitude of the signal and the cross talk component, and for detecting the ratio of the amplitude of said restored signal to said cross talk component, a plurality of balancing signal generation circuit means each coupled to one of said combined comparison and detection circuit means and to said respective reference audio channel for generating a balancing signal from said restored signal in said respective reference audio channel in accordance with said ratio, and a plurality of attenuating circuit means each coupled to one of said balancing signal generation circuit means and to audio channels corresponding to speakers flanking said reference channel speaker for applying the balancing signal to each of the flanking audio channels to attenuate the cross talk component of the signal resulting from the restored signal in each of the audio channels by applying a signal opposite in phase to said cross talk, whereby the cross talk component is eliminated for increasing the acoustic separation between the audio channels.
 5. A quadrophonic sound reproduction apparatus as claimed in claim 4 wherein said attenuating circuit means includes a phase inverter circuit for phase inverting the balancing signal and a mixer circuit connected to said phase inverter circuit to add the phase inverted balancing signal, to the cross talk component of the signal developed in the respective audio channels corresponding to flanking speakers, thereby eliminating said cross talk.
 6. A quadraphonic sound reproduction apparatus as claimed in claim 4, wherein said associated audio channel in a pair of audio channels is that audio channel into which said restored component of the signal from the reference channel does not ideally leak. 